The separation of various components found in liquids or gases may be effected in a multitude of processes, the techniques for effecting the separation including ultrafiltration or reverse osmosis. A particular example of the latter type of separation involves a desalination process in which water which is rendered potable or suitable for other purposes is obtained from sea water, contaminated water, brackish water or brine. This process is of especial value in areas of the world where the water found in the area is brackish or is saline in nature. The desalination of this water is necessary in order to provide large amounts of potable or relatively nonsalty water for industrial, agricultural or home use. The desalination of the water is effected by forcing the water through a reverse osmosis membrane whereby the purified water is passed through the membrane and recovered, while the contaminants or salts do not pass through the membrane, thus, in effect, being rejected by the membrane and recovered as the retentate.
A reverse osmosis membrane, in order to be utilized for such a purpose, must possess certain characteristics applicable to the process. For example, the membrane must have a very high salt rejection coefficient. In addition, another important characteristic and a problem which must be addressed when utilizing the membrane, is the ability of the membrane to be resistant to chlorine attack. Another important factor which is present in the use of a reverse osmosis membrane is that said membrane also possess a high flux characteristic, that is, the ability to pass a relatively large amount of water through the membrane at relatively low pressures. If a membrane possesses these desirable characteristics, it will be commercially feasible in its applicability to the desalination process.
Reverse osmosis membranes have been prepared and used from a wide variety of known polymeric materials. While many of these polymeric materials possess the ability of reducing the concentration of a solute to where the salt rejection capability is in excess of 98%, some do not possess the necessary flux rate whereby the volume of water which is required to be produced by the membrane per unit of membrane surface is sufficient for the application of the technology.
As was hereinbefore set forth, many prior U.S. patents describe various membranes which are useful in desalination processes. For example, U.S. Pat. Nos. 3,567,632, 3,600,350, 3,710,945, 3,878,109, 3,904,519, 3,920,612, 3,951,815, 3,993,625, and 4,048,144 illustrate various semipermeable membranes prepared from polyamides. Likewise, U.S. Pat. Nos. 3,260,691 and 3,480,588 disclose coating compositions which are obtained from the condensation products of aromatic primary diamines and aromatic tricarboxylic acid derivatives.
Inasmuch as the semipermeable membrane which is used for the desalination process should be relatively thin in nature in order to provide a desirable flux rate, it is necessary, in many instances, that the reverse osmosis membrane be composited or laminated on a porous backing support material. This porous support backing material should in itself possess certain characteristics which make it desirable for such a use. For example, the porous support material should possess pore sizes which are sufficiently large enough so that the water or permeate can pass through the support without affecting or lessening the flux rate of the entire composite. Conversely speaking, the pore size should not be large enough so that the thin composite semipermeable membrane will tend to fill up or enter into the pores, thus distorting the shape of the thin film membrane with the attendant possibility of rupturing the membrane, thus causing said membrane to lose its effectiveness in the reverse osmosis process.
In addition to the aforementioned U.S. patents, another U.S. patent, namely U.S. Pat. No. 4,277,344, discloses an interfacial synthesized reverse osmosis membrane. This membrane is prepared from a cross-linked interfacially polymerized aromatic polyamine which has been prepared from an essentially monomeric polyacyl halide and an essentially monomeric arylene polyamine. The patent indicates that the acyl halide which is employed as one of the components in the interfacial polymerization reaction should be free of interfering substituents. The patent refers to these substituents as moieties which are capable of interfering with intermolecular amine-carboxylic acid, amide-forming condensation reactions and points out that such interference is generally steric and/or chemically reactive. An example of steric interference or steric hindrance would be the location of a substituent other than hydrogen on a ring position adjacent to an amine group on the polyamine reactant. An example of chemical interference would be the location of an acyl halide reactive substituent on the polyacyl halide or an amine-reactive substituent on the polyamine. The patent further states that such chemical interfering substituents would lead to the formation of internal esters, internal amides, internal salts, or the like, or another possible consequence which would be attendant upon the performance of these moieties would be an unpredictable effect on cross-linked densities.
This patent also states that substituents on the aromatic nucleus other than the amine groups themselves are likely to detract from the performance of the ultimately obtained polymer, even the substituents which are as small as the methyl group, possibly having undesirable effects particularly when substituted on a ring position adjacent to the primary amine group. Thus, it is apparent that this patent teaches away from the presence of substituents on the aromatic ring of the polyamine to prepare a membrane such as that which is obtained when utilizing the components of the present invention.
In addition, the polymerized aromatic polyamine of this patent which may be depicted as follows: ##STR1## may undergo deliberate chlorination in which it is stated that the chlorination process properly removes unreacted aromatic polyamines to produce a compound depicted as follows: ##STR2## This deliberate chlorination of the membrane is required in order to improve the performance of said membrane in subsequent desalination operations. The patent also states that when the membrane of the reference was not fabricated under optimum amine and acyl halide concentrations, the salt rejection, even after the deliberate chlorination treatment, ranged from 88 to 97% and that even this loss of salt rejection capability can be minimized or avoided through modification of the polyamine; e.g., by acylation, alkylation, aldehyde condensation and other reactions which replace the amide hydrogen atoms with cross-links or other functional groups besides hydrogen. The focus of the patent therefore is on the modification of the amide portion of the membrane which is in contradistinction to the modification of the aromatic portion of the membrane which is produced according to the present invention. This modification of the aromatic portion of the membrane may be illustrated by the following formula: ##STR3## In this formula, it is to be noted that the chlorine substituent is on the aromatic ring portion of the polyamine, the dotted lines connecting the chlorine substituent to the ring indicating that the chlorine may be either in the 4 or 5 position depending upon the particular compound which has been chosen as one of the components of the desired membrane.
Another U.S. patent, namely U.S. Pat. No. 3,948,823, discloses the preparation of membranes from blends of preformed polymers. These membranes comprise a vastly different concept from the membranes of the present invention which are prepared by an interfacial condensation polymerization of reactive monomers on the surface of an inert support polymer. The polymers which are taught by this reference are not soluble in solvents which are compatible with the support polymers utilized in the present invention. Therefore, an attempt to cast a solution of a polymer as taught by this patent on the surface of a support polymer which is utilized in the present invention would result in the dissolution of the support polymer rather than in the formation of a composite membrane which is the present invention.
In contradistinction to what has been taught in the prior patents and this specification, we have now discovered that semipermeable membranes which possess the desirable characteristics of high salt rejection, good flux and high resistance to chlorine may be prepared in an interfacial polymerization reaction utilizing an aromatic polycarboxylic acid chloride and an aromatic polyamine in which the chlorine may be situated on a carbon atom adjacent to a primary amine group as the reactive components.